High molecular weight (HMW) kininogen is known to be a large plasma protein and cleaved by plasma proteinase kallikrein, then it generates four fragments in the blood coagulation cascade: heavy chain, bradykinin, fragment 1.2, and light chain. The fragment 1.2 has also been found in the basic protein fraction of bovine milk as a bioactive protein which promotes osteoblast proliferation. The milk basic protein has been shown to be a multi functional edible protein which promotes bone formation and inhibits bone resorption. In the present study, we purified the fragment 1.2 from bovine plasma and assessed it could promote osteoblast proliferation and posses the activity after pepsin digestion. Purified plasma HMW kininogen did not promote the proliferation, however, the kallikrein-cleaved HMW kininogen promoted the proliferation. The fragment 1.2, purified from the proteolysate, also promoted the proliferation. The pepsin digestion was performed according to the method of the assessment of allergenesity of genetically modified crops. After pepsin digestion, the fragment 1.2 generated resistant fragments and showed the promoting activity of osteoblast proliferation. These results suggest that the enzymatically-digested fragments of bovine HMW kininogen are able to be a naturally occurred active protein that promotes the bone formation by oral administration.